There has always been a need in society for verifying a person's identity for a variety of purposes. Modern day scientific technology has adopted the widespread use of computers and related mechanism for the purposes of giving credit, performing electronic funds transfer, and so forth. In all facets of the financial community including the retail industry, securities industry, banking and the like, sums of money, securities and/or materials are transferred between owners based on the reliance of one person on the purported identity of another. Electronic systems including various cryptographic instrumentalities together with secret identity numbers or keys provide a certain amount of security; however, the amount of security is predicted upon the degree of secrecy with which one is able to secure his own special identification key. Obviously, once a person's key is learned by another, presumably an unauthorized person, the other person may falsely assume his identity for a wide variety of electronic application.
Identity verification by means of written signatures has long been known in the art; however, most known systems have various shortcomings. Simply matching the appearance of two signatures is not satisfactory as expert forgers can usually duplicated the appearance of a person's signature as well as the person himself. The result of this is that when an expert forger is involved, even expert document examiners are frequently unable to discover that the signature is forged.
Recent developments in the field of automatic signature verification such as exemplified by U.S. Pat. No. 3,983,535 of Herbst et al and U.S. Pat. No. 4,128,829 of Herbst et al make the concept of personal identification via computer based signature analysis practical. The invention disclosed in U.S. Pat. No. 3,983,535 is based on the discovery that the accelerations of the stylus, which are proportional to the muscle forces exerted by the signer, are of predetermined consistent durations when forming particular strokes in a habitual signature. The nature of the process gives rise to the various distortions in the time axis; e.g., pauses between sections of the name, skipped strokes, decorative rubrics, and the like. Thus, the signal is marked by regions of high correlation of unknown duration separated by variable regions of low correlation. Accordingly, the invention in the U.S. Pat. No. 3,983,535 dealt with a method of regional correlation which registered these regions based initially on stylus contact and then shifting the regions individually to find the maximal of the correlation function weighted to penalize shifting. The results were then combined to make an overall verification decision.
The signature verification method disclosed in U.S. Pat. No. 3,983,535 was based on a single acceleration parameter of the signature dynamic, but as disclosed in U.S. Pat. No. 4,128,829, an even greater discrimination in the verification operation is possible using two orthogonally disclosed (e.g., X and Y axes) acceleration components together with the pressure patterns which are produced during the writing of the signature and utilizing all three of these individual parameters in the correlation operation. The invention disclosed in U.S. Pat. No. 4,128,829 retains the concept of segmenting the sample and reference signatures, correlating individual segment pairs utilizing a series of successive shifts to obtain the maximum possible correlation, weighting the correlations, and finally combining the individual correlation statistics for all segments. An example of a pen system is disclosed in U.S. Pat. No. 4,142,175 of Herbst et al. This pen produces electrical signals proportional to accelerations along the X and Y axes and an electrical signal proportional to the pen point pressure along the Z axis. The acceleration along the X-axis (Y-axis) is known as an acceleration component. The Z-axis is essentially parallel to the axis of the pen. The most recent development is writing instruments is described in U.S. Pat. No. 4,513,437 to Chainer et al. This patent discloses a pressure and acceleration sensing instrument. The pressure sensing element is axially mounted in the writing instrument. The accelerometer structure comprises bimorph piezoelectric members supported at one end, which are appropriately interconnected to produce two orthogonal acceleration components Ax and Ay.
According to the Herbst et al procedure, reference acceleration and pressure signals are stored in memory in the electronic computer. Actually, as will be understood by those skilled in the art, digital representations of the acceleration and pressure signals are stored, and the acceleration and pressure signals produced by the pen when used to write a signature are also digitized so that all the arithmetical processing is performed digitally. In a typical system, when a customer opens an account, a signature acquisition feature on a computer terminal prompts the customer to sign his or her name several times. This produces signature data that is transmitted to the computer which selects the reference signals that are stored. Both the reference signals and the signals from the pen produced by a person whose signature is to be verified are segmented as a function of pen lifts which are detected by the pressure signal becoming zero as described in U.S. Pat. No. 4,553,258 Chainer et al. Pen lifts are critical to good correlation scores as they represent reproducible timing marks in the signature. The segmented acceleration and pressure signals from the pen are then compared with the corresponding reference acceleration and pressure signal segments using the correlation algorithm disclosed in U.S. Pat. No. 3,983,535 to Herbst et al.
A segment shifting technique of maximize the correlation for the segments of acceleration signals is disclosed in U.S. Pat. Nos. 4,562,592 and 4,553,259 of Chainer et al, assigned to the same assignee as the present application and for which the issue fee has been paid. The above application (Ser.No. 567,201) and U.S. Pat. Nos. 3,983,535, 4,128,829, 4,142,175 and 4,513,437 are hereby incorporated herein by reference.
The latest development in a signature verification methodology (U.S. Pat. No. 4,128,829) represents an improvement over past practices by applying correlation to pressure and to two orthogonal acceleration components.
The aforementioned signature verification methods have complicated logic structures to decide whether to accept or reject a sample signature. The origin of the problem is due to the inability of any one measure to distinguish reliably between verify and forgery signatures. For example, a former method uses three measures that we will call m1, m2 and m3. Let us assume that ml is the best discriminator and m2 is second best. This method starts with a weak test on m3. Any signatures rejected by m3 would surely be rejected by m1 or m2 so this test only serves the function of saving computation time. m1 is then examined with three possible outcomes; (1) accept, (2) reject or (3) test m2. If test m2 is selected then M2 makes the final decision which is not desirable since m2 is not as good a discriminator as m1. Interchanging m1 with m2 in the above structure doesn't help because m2 passes on too many forgeries that m1 cannot distinguish. In addition, these methods do not allow one to easily trade increase (decreased) verify error rates for decreased (increased) forgery error rates. Such an ability would surely expand the possible applications for a signature verification system.
There is, therefore, a need for a signature verification method with a simple decision logic structure which is easily tunable to particular forgeability requirements. Further, there is a need for a verification method wherein one measure may dominate over another on an individual signature basis, depending upon how reliable such a measure is in distinguishing between forgeries and verifies (true signatures).